In use, pressure sensors can drift in their calibration outside of their desired accuracy for the application. A sensor might also suffer partial or total failure. Diaphragm based pressure sensors are known to suffer from calibration drift over time. This can manifest as a calibration-slope drift or as a zero-pressure value drift. The causes are often specific to the application. Most calibration-slope drift problems are related to changes in the diaphragm's mechanical properties. Many sensors attempt to use very stable materials to avoid calibration drift, such as Silicon-on-Sapphire (“SOS”), which is a hetero-epitaxial process that consists of a thin layer of silicon grown on a sapphire (Al2O3) wafer. A source of zero-pressure value drift, is stress relief from the diaphragm mounting condition. Careful packaging is directed toward this problem. Compensation for temperature calibration offsets has been developed to a high degree, but little has been done to address changes in the mechanical properties of the diaphragm.
Building a very stable sensor based on careful selection of materials, careful mechanical design, and careful selection of processing techniques is admirable. Statistically, in a given critical application, it is still necessary, however, that the accuracy of the sensor be determined in-situ to allow confidence in the performance of the system in which it is embedded. This calibration is typically performed by attaching a known, external, reference pressure transducer in parallel to the sensor to be verified.
There are a number of causes for changes in the diaphragm's properties. Some examples include, but are not limited to, (1) annealing of the diaphragm material through temperature cycling or large temperature excursions, resulting in changes to the elastic modulus of the diaphragm, (2) etching of the diaphragm by the fluid being measured, resulting in a thinning of the structure and a change in diaphragm stiffness, (3) chemical reaction of the diaphragm with the fluid being measured, resulting in a change in the nature of the material on the pressure side, which might result in a composite structure with a different effective modulus and stiffness, and (4) deposition of material dissolved or suspended in the fluid being measured, resulting in a change in the nature of the material on the pressure side, which might result in a composite structure with a different effective modulus and stiffness.
There is a need in the art for a sensor that is capable of self-diagnosis and determination, self-calibration correction and confidence reporting without coupling to an external reference. If a sensor could self-determine the mechanical properties of the diaphragm for reasonable changes, it could correct the slope calibration and thus provide a longer service life with a stated accuracy. If the measured changes are larger than reasonable, the need for replacement could be detected in-situ. The present invention provides a sensor that could detect and correct for the first three cases mentioned above, and detect the fourth case. This would allow the sensor to effectively determine its own maintenance schedule, reducing the overall cost of maintenance and reliability for a system containing a number of such sensors. More importantly, it would help to identify sensors approaching degradation failure and thereby reduce overall down time for the system that would be caused by catastrophic failure of the sensor.